The present invention generally relates to acoustic systems and more particularly to underwater sound transmitting or receiving systems having the unique property of providing a scannable directional substantially constant beamwidth diffraction pattern or beam over a wide band of frequencies.
A unique feature of the present invention as compared to other acoustic antenna arrays is the use of fully directive acoustic lens antennas each with an appropriate retina for scanning as the primary array antenna elements. This feature together with appropriate time delay devices eliminate the usual dependence of scanning delay requirements on wavelength or frequency and makes the scanning properties independent of frequency.
A second unique feature of the present invention is the use of a network of filters, switches, amplifiers and time delay elements with the array of acoustic lenses in such a way as to make the beamwidths of the beams formed by the array independent of frequency or constant in beamwidth over very large changes in frequency.
Many prior art devices in the underwater field have addressed themselves firstly to beam forming with array antennas and secondly to the problem of providing wide band frequency response so that maximum sound pressure level remains uniform over a wide range of frequencies. However, the only prior art devices known to have addressed themselves to the antenna design problem of maintaining a scannable substantially constant beamwidth directional diffraction pattern or beam over a wide range of frequencies are those described by Robert L. Sternberg in U.S. Patent Application Ser. No. 784,186, Frequency Independent Acoustic Antenna, and U.S. Pat. No. 4,068,082, Combination Acoustic Filter Plate and Liquid Lens. A primary limitation of these prior art devices is that their construction limits their range of practical application to problems requiring an antenna no more than a few feet or a few meters in diameter as otherwise the dense construction of the filter plate in the devices would make the system excessively heavy.